This invention relates generally to nuclear reactors and more particularly to a guide tube centering device for use in nuclear reactors during fuel assembly handling operations.
A nuclear reactor produces heat by the fissioning of nuclear materials located in fuel rods which are assembled in fuel assemblies. A plurality of these fuel assemblies comprises a nuclear core, and the nuclear core is situated in a nuclear reactor pressure vessel. In pressurized water nuclear reactors, the nuclear core is generally of the open lattice variety in which the individual fuel assemblies are located adjacent to one another without any structural separation.
During the course of reactor operation certain fuel assemblies tend to burn up the fissile material more rapidly than other fuel assemblies. In order to maximize the life of the core, it is a customary practice in nuclear reactors to shuffle the fuel assemblies. By shuffling, it is meant that the fuel assemblies which burn up more rapidly are removed from the core, and the fuel assemblies which had been positioned in the slower depletion regions are moved to the locations of higher material depletions. By so shuffling the fuel assemblies, the useful life of the individual fuel assemblies is prolonged.
During replacement, the depleted fuel assemblies are removed from the core to a storage container, and new fuel assemblies are inserted into certain predetermined positions in the nuclear core. During both shuffling operations and replacement operations, it is necessary for the individual fuel assemblies to be vertically raised out of the nuclear core. To accomplish this vertical movement, a fuel handling mechanism is generally positioned above the pressure vessel, and is inserted downward to engage the fuel assemblies, and is then reversed to vertically move the fuel assemblies out of the core.
In pressurized water reactors, and other nuclear reactors utilizing open vessel refueling techniques, the pressure vessel closure head and the upper internals structure are removed prior to refueling operations. A manipulator crane is then positioned vertically above the nuclear core. The manipulator crane generally has a downward vertical extension commonly called a mast. This mast is generally comprised of two parts: a stationary mast, which extends vertically downward to an elevation above the open reactor vessel; and a telescoping guide tube which extends from the stationary mast to the fuel elements. A fuel handling mechanism is generally installed within the guide tube, and this fuel handling mechanism contacts the individual fuel assembly.
In an open lattice core, the individual fuel assemblies are located approximately 0.04 inches (0.1016 cm.) apart. Because of the close proximity of adjacent fuel assemblies, the guide tube, and its related fuel handling mechanism, must maintain an exact alignment with the fuel assembly to be removed. A misalignment of 0.5 inch (1.27 cm.) can cause an unstable connection between the fuel handling mechanism and the fuel assembly, and possibly result in damage to adjacent fuel assemblies. Thus, it is important to maintain vertical alignment between the guide tube and its fuel handling mechanism, and the individual fuel assembly.
In conventional nuclear reactors, misalignment can occur because of the nature of the telescoping guide tubes. Rollers must be inserted into the stationary mast to enable the guide tube to descend downward approximately 40 feet (13.92 meter) into the pressure vessel. These rollers are generally positioned around the inside of the stationary mast so as to maintain a constant pressure against the guide tube. However, the rollers may possess different tensioning, and result in a deflection of the guide tube. This deflection of the guide tube then causes a misalignment of the fuel handling mechanism, and could result in a fuel handling accident. Additionally, misalignment may occur because of refueling cavity water clarity problems.
Because of the reluctance of segments of the population to have nuclear reactors constructed in their geographical vicinity, the possibility of building nuclear plants in the oceans is being seriously investigated. The electricity generated by these offshore power plants would be transferred to land by high voltage transmission lines. These power plants would be constructed on land, on movable barges, and would then be towed to their desired position where they would be secured to the ocean floor. These offshore nuclear power plants would then provide electricity to large geographical areas without alienating segments of the population who do not want nuclear installations in their geographical vicinity.
The aforementioned problem of maintaining alignment during fuel handling operations is one of the many problems associated with designing an offshore nuclear installation. In addition to the misalignment problems previously mentioned, an offshore nuclear installation has alignment problems resulting from the forces imparted to the installation by wave movements generated in the ocean. Even through anchored to the ocean floor, the installation will nevertheless be subjected to roll caused by wave motion. Although this roll may seem minimal, generally one-half degree of less, it is a serious concern when considered in conjunction with the fuel handling operation. A one-half degree roll at the manipulator crane will cause a serious deflection at the end of the guide tube, some 40 feet (13.92 meter) away when extended, and could cause misalignments greater than the permissible 0.5 inches (1.27 cm). Therefore, it is necessary to provide a centering device to maintain alignment between the guide tube, and its associated fuel handling mechanism, and the individual fuel assemblies during refueling operations.